For a digital mobile unit communication system, there are proposals about an adaptive modulation method such as HDR (High Data Rate) in recent years. A communication using a conventional HDR will be explained with reference to FIG. 1 below. FIG. 1 is a schematic view showing a downlink slot format used for communications using an HDR.
First, a communication terminal apparatus demodulates pilot (PL1 and PL2) sections of a received signal and detects an SIR. The communication terminal apparatus then decides a modulation method that allows the reception quality of data sections to satisfy a required quality based on the detected SIR and decides the modulation method to be requested to a base station apparatus.
A specific example of a method of deciding the modulation method will be explained with reference to FIG. 2. FIG. 2 is a schematic view showing a method of deciding a modulation method in a communication using a conventional HDR.
In FIG. 2, when the SIR of a pilot section is lower than threshold 1, even if the communication terminal apparatus receives data to which 16 QAM or 64 QAM is applied, the reception quality of the data section does not satisfy a desired BER (10−3). Therefore, the communication terminal apparatus selects QPSK as the modulation method to be requested to the base station apparatus.
Furthermore, when the SIR of the pilot section is between threshold 1 and threshold 2, even if the communication terminal apparatus receives data to which 16 QAM is applied, the reception quality of the data section can satisfy the desired BER. Thus, the communication terminal apparatus selects 16 QAM as the modulation method to be requested to the base station apparatus. Furthermore, when the SIR of the pilot section is higher than threshold 2, even if the communication terminal apparatus receives data to which 64 QAM is applied, the reception quality of the data section can satisfy the desired BER. Thus, the communication terminal apparatus selects 64 QAM as the modulation method to be requested to the base station apparatus. These are specific examples of the method of deciding the modulation method.
Then, the communication terminal apparatus notifies the decided modulation method to the base station apparatus.
On the other hand, the base station apparatus performs scheduling based on the modulation method notified from each communication terminal apparatus and sends data to which the modulation method notified from this communication terminal apparatus is applied to the communication terminal apparatus decided by the scheduling.
However, movements of a communication terminal apparatus cause fading in a mobile unit communication system. In a fading environment, even if there is no change in the reception quality of a pilot section (pilot signal), a Doppler frequency of fading (fd) may cause the reception quality of the data section (received data) to deteriorate, which results in a large difference between the reception quality of the pilot section and the reception quality of the data section.
More specifically, since the time zone occupied by a pilot section in a slot is short as is apparent from FIG. 1, the reception quality of the pilot section in the communication terminal apparatus is favorable even in a fading environment. On the other hand, since the time zone occupied by a data section in a slot is long, in a fading environment, a phase rotation occurs in the data section received by the communication terminal apparatus due to the fading. This causes the reception quality of the data section in the communication terminal apparatus to deteriorate, which results in a large difference between the reception quality of the pilot section and the reception quality of the data section (that is, the reception quality of the data section falls below the reception quality of the pilot section in a fading environment).
As a result, when the communication terminal apparatus decides a modulation method to be requested to the base station apparatus based on the reception quality measured in pilot section, the reception quality of the data section in the communication terminal apparatus fails to satisfy the required quality.
As shown above, the conventional adaptive modulation method has a problem that the quality of received data in a communication terminal apparatus falls below the desired quality in a fading environment, having difficulty in performing highly efficient and high quality data communications.